A hologram is a physical structure that diffracts light into an image. The term ‘hologram’ can refer to both the encoded material and the resulting image.
A holographic image can be seen by looking into an illuminated holographic print or by shining a laser through a hologram and projecting the image onto a screen.
Other methods of projecting and reflecting images are often described as holographic – or even misleadingly holograms, because they have an optical presence, spatial quality or iridescent colors.
In the past there was a clear distinction between holograms and other visual media. However as technology evolves there are more hybrid media that use some of the principles of holography, or created spatial images that look like holograms.
Not a hologram (but often called holograms) are Pepper’s ghost projections, which use a partially reflective surface to mix an image with the scene beyond. Think about looking out a window at night time and seeing the reflection of the room you in overlaid on the scene outside. John Henry Pepper demonstrated this technique in the 1860s with it being used to overlay visual elements –often ‘ghost’ character – onto a physical set or stage.
Augmented reality headsets draw on the Pepper’s ghost technique to create virtual images that appear in space. Headsets are able to deliver a slightly different view to each eye creating the illusion of depth through stereo-parallax. More advanced AR systems use Holographic Optical Elements and Wave Guides to shape the reflected light so the images optically focus in space.